This invention relates to offshore platforms and drilling processes. More specifically, the invention is related to low-heave or heave-constrained buoyant platforms, such as tension leg platforms used to drill into and produce fluids from offshore hydrocarbon resources.
Some offshore resource recovery activities, e.g., withdrawal of hydrocarbon fluids from a subsurface reservoir through underwater tubular risers to surface facilities, are typically accomplished using an offshore platform. The offshore platform supports at least a portion of the surface facilities and well tubulars while withstanding waves, wind, and other offshore environmental conditions. The offshore platform typically includes an above-water deck or other working platform for the placement of at least a portion of the resource recovery facilities. For offshore platforms located in shallow water depth locations, a generally rigid tower structure supports the working platform and is typically fixed to an underwater or seafloor anchor or foundation. For offshore platforms located in deeper waters, e.g., offshore platforms located in waters having a depth typically exceeding about 500 to 2000 feet (or about 150 to 600 meters), this type of fixed tower structure is typically not cost effective, and buoyant platforms or other types of resource recovery facilities are more likely to be used.
Drillships or other buoyant platforms typically use one or more buoyant hull portions to support the risers and other resource recovery facilities, but buoyant platforms are susceptible to significant positional variations due to wave and wind actions, e.g., up and down position variations or heave motions. A low-heave buoyant platform can be achieved by using various design options, e.g., semi-submersible spars, deep draft flotation elements, and increased skin friction devices. However, some amount of heave motions must still be accommodated by low-heave platforms.
Tension leg platforms (TLPs) are a type of heave-constrained offshore structure that also incorporates one or more buoyant portions, but includes tension cables, tubulars, or other heave restraint means attached to the platform and extending generally downward toward a seafloor or other underwater anchor, typically secured by subsurface piles. These substantially vertical or heave restraint means are sometimes called tendons. The tendons are typically placed in tension, e.g., to provide resistance to the buoyant or heave forces on the submerged hull portions at essentially all times during resource recovery operations. When compared to some other buoyant or semi-submersible platforms, the tendons of a TLP greatly limit heave motions due to waves and other environmental forces that tend to vertically displace the buoyant portions of the TLP, but do not entirely eliminate all platform motions.
Unless otherwise restrained or controlled, significant lateral offset motions of the working platform of a TLP are typically still present in deep water applications due to the variations in wind, waves, tides, currents, and other environmental forces. The tendons of a TLP may provide some added restraint and stability in pitch, roll, or other motions, especially if relatively stiff tendons are used, but some pitch, roll and other motions of a TLP are typically present. The tendon-restrained TLP in deep water moves somewhat like a parallelogram-like structure with pinned corners when the TLP is exposed to lateral offset forces. With significant lateral forces and platform offset motion, the upper portion of the TLP xe2x80x9csetsdownxe2x80x9d (i.e., the upper buoyant portion tends to be pulled downward by the tendons when the upper portion is laterally offset), thus lowering the position of the partially submerged buoyant portion relative to the waterline. In addition to increases in wave height and/or mean water levels due to extreme environmental conditions, this lateral offset and lowering tends to create increased buoyant forces and increased riser tension.
Because extreme environmental conditions might impose unacceptable tensile loads on a riser fixedly attached to a heave-constrained platform, dynamic riser connectors and tensioning equipment have typically been used. The dynamic riser tensioning equipment allows relative vertical motion between the riser and platform while maintaining a generally constant tensile force on the riser. Such equipment can comprise hydraulic cylinder(s) with a piston and rod end attached to the riser and the cylinder attached to the platform so that a substantially constant tensile force is applied on the riser generally independent of the position of the piston(s) within the cylinder(s).
However, flexible pipe connections and dynamic riser tensioners typically provide relatively little additional lateral or other motion constraint when compared to fixed connections. A platform with flexible pipe or dynamic riser connections may require additional tethers, mooring cables or other means for providing further resistance to the lateral, pitch, roll or other motions. In addition, dynamic riser connections limiting the tension on the risers of a TLP may transfer excessive peak and/or jerk loads onto the tendons when the TLP is exposed to extreme (e.g., high wind and wave) environmental conditions or a failure of the dynamic riser connection. Dynamic riser tensioners can also add significant cost and complexity to the riser system.
One embodiment of the present invention provides a substantially fixed position, but adjustably located riser connector and a process for adjustably locating and fixedly connecting a riser to a buoyant platform. The preferred process positions and preloads the riser/connector, fixedly supports the riser, removes the preload, and allows later repositioning of the connector without the use of a drill rig. The substantially rigid riser connection provides increased stability and motion restraint while the connector preloading and repositioning capabilities limit riser and/or tendon loads to safe levels. The preferred riser connector comprises a nut threadably attachable to a riser length adjustment spool, means for adjusting the axial position of the nut relative to the riser, means for pre-loading the nut and/or riser, and means for removing the preload. The means for pre-loading and removing the preload is preferably by pressurizing and depressurizing a plurality of fluid or hydraulic actuators contacting the nut. The means for position adjusting is preferably a motor-driven support ring rotator and a plurality of support bars connected to the nut. The support bars preferably have overall length dimensions comparable to the length of the hydraulic actuators, allowing one or more hydraulic actuators to be relocated and to temporarily support a portion of the riser tensile load.